By way of introduction, in the hip implant field, two main arts may be delineated: on one hand is conventional hip replacement arthroplasty, where all of the femoral neck and head is replaced with a prosthetic implant; on the other hand is articular resurfacing, also known as conservative hip replacement, in which event only the outer articulating portion of the femoral head is replaced with a much thinner, bone-preserving implant. In each instance, the procedure may be limited to a hemiarthroplasty by replacing only the femoral side of the hip, leaving the implanted femoral component to articulate with the acetabulum's own socket; or it may be extended to a total joint arthroplasty by also replacing the socket side of the joint with a matching acetabular component implant.
Femoral components with ceramic heads are known for conventional hip replacement.
And, interchangeable, modular implants are not routinely employed for conventional hip replacement.
conventional hip replacement itself may be associated with several drawbacks, among these, the great amounts of bone which must be resected so as to accommodate the femoral component. See, e.g., the brochure from BioPro, Inc., "PSL Total Hip Replacement System." In addition, a less than desirably natural postoperative gait may result.
With respect to the pertinent, conservative resurfacing art, Townley, Charles O., M.D., Orthopedic Clinics of North America, Vol. 13, No. 4, October 1982, "Hemi and Total Articular Replacement Arthroplasty of the Hip with the Fixed Femoral Cup," reported on detailed information concerning the design rationale and surgical technique for the total articular replacement arthroplasty (TARA) conservative hip procedure. Subsequent thereto, the component design and implantation technique have remained essentially unchanged except for the development of improved instrumentation to remodel the femoral head, and the conversion to porecoated, uncemented implants in conjunction with a self-locking, metal-backed acetabular component in 1985. See also, the brochure from BioPro, Inc., "The Biopro Tara Surgical Procedure."
Prior femoral head hip resurfacing components have been machined from metallic materials, to include cobalt-chrome, stainless steel, and titanium.
And, with respect to the resurfacing art, femoral components have been designed as one-piece, inseparable stem and cup items.
Some drawbacks in the foregoing articular resurfacing systems include, first, a less than desirable smoothness (micro finish) of the articulating surface of the metallic prosthesis, which is typically a four to five millionths of an inch micro finish; and second, the presence of a one-part femoral component.
The first drawback mentioned above with respect to the articular resurfacing art is reflected in the incidence of latent, friction-induced postoperative complications which adversely impact survivorship longevity of the arthroplasty as a consequence of the limitation in the obtainable surface smoothness of the metallic femoral resurfacing component in current use. In the experience of the inventor, the predominant, singular complication in the case of a metal-on-polyethylene Total Articular Replacement Arthroplasty relates to a sequential, friction-induced scenario of polyethylene wear degradation; the consequent proliferation and dissemination of wear debris which accesses the supporting bone-prosthesis interface, which, in turn, precipitates a debris-induced foreign body reaction and an associated degree of periprosthetic osteolysis, sufficiently severe to culminate in an implant failure rate on the order of fifteen percent with an average post-operative failure time of some ten years. In case of femoral hemiarthroplasty, the procedure is not infrequently followed by variable degrees of friction-induced pain and latent acetabular wear protrusio associated with the unreplaced socket articulation, which, in either event, may require additional surgical intervention.
The second drawback mentioned above with respect to the articular resurfacing art is reflected in the adverse economic impact associated with the pre-fixed, inseparable, non-modular arrangement of the two-part prosthesis. The same requires a redundant, costly on-shelf inventory of implants to accommodate the individual anatomic variance between the size of the femoral head and the configuration of the medullary canal of the proximal femur in patients consigned for resurfacing arthroplasty. Economic considerations aside, the concept of a one-piece, non-modular system with its fixed combinations of femoral head and stem sizes may not fit the anatomical needs of a given patient relative to one or the other of its stem and head (cup).